In Japanese Patent Publication No. H10-203338, a brake-by-wire vehicle brake control device is proposed which has four pumps respectively for the four wheels of a vehicle. Two of the four pumps are located in a common conduit system for two of the four wheels and are driven by a common motor. The other two of the four pumps are located in another common conduit system for the other two of the four wheels and are driven by another common motor.
In the case that the vehicle travels on a cross-border road on which the vehicle experiences different friction coefficients μ at its left and right wheels, brake forces with different strength have to be applied to left and right wheels in an anti lock braking system (hereinafter referred to as ABS) control. More specifically, the vehicle brake control device increases the W/C pressure for a wheel on a high friction section of the cross-border road and decreases the W/C pressure for a wheel on a low friction section of the cross-border road, where the high friction section is a section of the cross-border road having a higher friction coefficient and the low friction section is a section of the cross-border road having a lower friction coefficient.
An ABS control device having an ordinary hydraulic circuit generates the W/C pressures by guiding a brake fluid pressure (hereinafter referred to as an M/C pressure) generated at the master cylinder (hereinafter referred to an M/C) to the W/Cs. The conventional ABS control device also includes pairs of an increase valve and a decrease valve, the pairs installed respectively to the wheels, wherein the increase valves are respectively for increasing the W/C pressures and the decrease valves are respectively for decreasing the W/C pressures. In decreasing the W/C pressure for the wheel on the low friction section, the above conventional ABS control device allows brake fluid to flow into a reservoir to which a low pressure is applied. Therefore, depressurization is executed without interference from increase of the W/C pressure for the wheel on the high friction section.
However, in the case that two pumps are installed to a common conduit system and are driven by a common motor, increase of the W/C pressure at the wheel on the high friction section causes both of the pumps to discharge the brake fluid and accordingly interferes decreasing of the W/C pressure at the wheel on the low friction section. Therefore, the depressurization may become insufficient.